Summary of work: We have reported that there is an increase with age in the activity of a mitochondrial DNA glycosylase/endonuclease which is specific for 8-OH-deoxyguanosine (8-OHdG). In liver mitochondrial extracts from rats,there was a highly significant increase in activity between 6 months and 23 months of age, with the activity at maximum at 12 months. The same pattern was seen in extracts of heart mitochondria. By contrast, two other mitochondrial enzymes of DNA metabolism which are not specifically involved in the repair of oxidative damage, uracil DNA glycosylase (mtUDG) and AP endonuclease, were found to be unchanged with aging. This increase in mitochondrial DNA repair with aging is also seen in mice. Here, we again observe an increase in DNA repair with age for the mitochondrial DNA, while the nuclear DNA repair slightly decreases with age. It is likely that the mitochondrial base excision repair glycosylases are induced by DNA damage, since 8OHdG has been shown to accumulate in mtDNA with age. We have further investigated the role of the oxoguanine DNA glycosylase 1 (OGG1) in mtDNA repair by using tissues from mice that are defective in this enzyme. We found that liver mitochondria from the OGG1 knockout mice have no detectable 8OHdG incision activity, showing that this mitochondrial activity is encoded by the same gene as the nuclear enzyme. We also found that mtDNA from the knockout mice accumulate 9 times more 8OHdG than wt animals. In contrats, nuclear DNA from the null mice have only two time more 8OHdG modifications than controls. These results suggest that OGG1 plays a crucial role in the repair of oxidative damage in mitochondria and is probably the only 8OHdG glycosylase in these organelles. Repair of oxidative DNA damage is carried out by the base excision repair (BER) system. BER can occur through two pathways, long and short patch repair. We investigated the repair patch size during repair of uracil in mitochondria from human cells. Our results show that uracil in DNA is repaired solely by the short patch pathway in mitochondria, while the long patch pathway is extensively used in the nuclear repair of the same lesion. Caloric restriction is a major therapeutic intervention against age associated disease. We are studying DNA repair of oxidative DNA lesions in calorically restricted mice to determine where DNA repair is affected by diet. We are also subjecting knockout mice, defective in specific DNA repair genes to caloric restriction as another approach to determine whether DNA repair plays a role in this process.